In recent years many advances have been made in private branch exchange (PBX) telephony. Small PBX systems which may handle calls from less than ten trunk lines have become economically feasible for small business operations, and very sophisticated private automatic branch exchange (PABX) systems have been built which allow connections to PABX stations from outside trunk lines in the order in which the incoming calls on the trunks were received.
In large PABX systems, such as airline reservation centers, it has been desirable to provide each station within the exchange that is receiving incoming trunk calls with information as to the origin of the call being handled. For example, it is often the case that an airline reservation center is located in one city but trunks from a large number of cities are connected to the PABX system of the reservation center. Heretofore such information to station operators has been provided by relatively costly displays such as cathode ray tube alpha-numeric displays.
On smaller PBX installations, only a limited amount of information has been available from the signal conditions on individual branch lines, such as a light indicating that the line is in use, or a "hold" condition exists on the line.
In the past there has not been an economical way of providing a relatively small PBX or PABX system with information which indicates the source of an incoming call. Furthermore, PABX systems have not been able to provide an economical display at each station which gives information as to origin of incoming calls, total system incoming call backlog and other information which will allow the station operator to respond appropriately to a caller.